This invention relates to semiconductor wafer processing, and more particularly to removal of damage to the wafer by use of laser annealing.
The processing of silicon to a form useful in fabricating integrated circuits requires use of machines such as saws, grinders, and polishers, which physically alter the shape of the silicon itself by tearing away part of it. In this tearing away, physical damage is done to the remaining surface, and even after attempts to smooth the surface by etching and polishing, small cracks and crevices exist in the material surface. These defects have a negative effect on the electrical performance of any device subsequently fabricated on the surface.
The first step in the process of wafer formation is that of sawing a silicon crystal into thin wafers. These wafers are then put on holding fixtures and edge ground to remove the roughness that may be on the edge of the wafers and to round the wafer. Next, a lapping operation removes as much saw damage to the surface as is possible, and at the same time, planarizes the surface of the wafer. The stress relief etch step is then performed, which attempts to remove micro-splits, or small stress cracks. The wafer surface is then given a final polish which removes a minute amount of the surface as a final effort at physical removal of surface damage.
Typically, each step may remove some surface damage, but may also cause damage. Therefore, some means for removal of damage remaining on the surface is needed.
It is an object of the present invention to remove minute damage to the surface of a wafer. Another object of the present invention is to denude the surface layer, of a wafer, of oxygen.
This invention may be used to eliminate a step or steps in the typical wafer process, or used between steps of the process.
The use of laser annealing to repair crystal lattice damage in silicon, caused by ion implant, has been demonstrated by Gibbons (Electronic Design, Jan. 79, pp. 23-25). The damage caused by ion implant is largely confined to a depth of less than 1 micrometer. The surface layer then does not require melting, only sufficient heat to let the lattice structure reform.
Laser annealing involves the focusing of a laser beam onto a spot, and sweeping this spot across the surface of a wafer. The concentration of enough power on a spot can cause melting of the surface layer of silicon. The desired effect on a surface may be achieved by manipulation of spot size and scan rate of the spot.
The present invention utilizes a laser to melt the surface of the wafer. To achieve a melt depth of approximately 0.5 to 1 micrometer, a laser having a wave-length of slightly more than 1 micrometer may be used, such as the Nd.sup.3+ :YAG type, which has a wave-length of 1.06 micrometers. For a deeper melt of approximately 10 micrometers, a laser having a longer wave length must be used, such as the CO.sub.2 type laser, which has a wave-length of 10.6 micrometers. The laser used is either a pulsed or CW type of laser, and is focused to a spot size of 50 micrometers in diameter. The pulsed laser has a pulse duration of 50 to 150 nanoseconds typically with a pulse repetition rate of 0.05 to 50 Hz. This results in the necessary power density of 50 to 200 MW/cm.sup.2. The CW laser must be operated in the Q- switched mode with pulse repetition rate of 1 to 10 KHz, and must have an average output power of 10 to 20 watts in order to achieve the necessary power density. The melting and subsequent substrate oriented recrystallization allows for removal of shallow surface damage to a depth of 1 micrometer, or removal of deeper surface damage up to 10 micrometers, and generally minimized the amount of material removal during wafer fabrication.
The present invention has an added feature in the preferred embodiment, whereby annealing is conducted with the wafer in a partial vacuum of approximately 10.sup.-4 mm of mercury, causing the surface film to be denuded of oxygen. A mostly oxygen-free surface film is superior for VLSI fabrication since no oxygen precipitation can occur due to supersaturation effects. Oxygen precipitation and related effects in the depleted region of MOS and CCD circuits are known to enhance dark currents. However, oxygen precipitation in the bulk substrate will enhance device performance by acting as a sink for fast diffusing impurities and point defects.